Sacred Hill

Folklore

St. Brigid was seemingly born nearby and graveyard/abbey is where St. Brigid became a nun.
Underneath the hill was the Sli of Brí Ele.

From Mythic Ireland
"In early literature, Ele is represented as a maiden and goddess of awe-inspiring beauty. Every year one of Finn's young men went out from the Hill of Allen to meet her and was lost for ever, in her síd beneath Croaghan Hill.
The Dindshenchas stated that the source of the River Shannon was under Brí Ele at a well called Linn Mna Feile 'the Pool of the Modest Woman'."
It is also associated with the underworld and being sucked into the hill. The cairn on top is meant to have been the entrance to a volcano.
Brigits sister (part of the triple goddess), Begoibne had her smithy workshop beneath the hill and built beautiful cauldrons.
He also states that winter solstice sunrise is visible over Cloghan hill from the Hill of Uisneach.